Strong vs Weak Entity Set:

• Strong Entity: Exists independently, has primary key. Represented by single rectangle. Example: Customer (cust_id), Car (car_id).

• Weak Entity: Cannot be uniquely identified by own attributes alone, depends on strong entity, no PK, has partial key (discriminator). Existence dependent, total participation. Represented by double rectangle, identifying relationship double diamond. Example: Accident depending on Car, Dependent depending on Employee, Loan Payment depending on Loan.

Difference table: Strong has PK, existence independent, single rectangle; Weak needs owner, partial key + owner PK forms PK, double rectangle, always total participation.

E-R example: Employee (strong) -- Employs -- Dependent (weak, attributes: dep_name (partial key), age, relationship). Dependent PK = emp_id + dep_name. Relationship is identifying, cardinality 1:N, double lines from weak to relationship.